Patients with kidney failure or partial kidney failure typically undergo hemodialysis treatment in order to remove toxins and excess fluids from their blood. To do this, blood is taken from a patient through an intake needle or catheter which draws blood from a blood vessel located in a specifically accepted access location (for example, a shunt surgically placed in an arm, thigh, subclavian, etc.). The needle or catheter is connected to extracorporeal tubing that is fed to a peristaltic pump and then to a dialyzer which cleans the blood and removes excess water. The cleaned blood is then returned to the patient through additional extracorporeal tubing and another needle or catheter. Sometimes, a heparin drip is located in the hemodialysis loop to prevent the blood from coagulating. By way of background, as the drawn blood passes through the dialyzer, it travels in straw-like tubes within the dialyzer which serve as semi-permeable passageways for the unclean blood. Fresh dialysate solution enters the dialyzer at its downstream end. The dialysate surrounds the straw-like tubes and flows through the dialyzer in the opposite direction of the blood flowing through the tubes. Fresh dialysate collects toxins passing through the straw-like tubes by diffusion and excess fluids in the blood by ultra filtration.
It is known in the art to use an optical blood monitoring system during hemodialysis, such as the CRIT-LINE® monitoring system which is sold by the assignee of this application. This blood monitoring system uses optical techniques to non-invasively measure in real-time the hematocrit level of blood flowing through a hemodialysis system. In this system, a sterile, single-use blood chamber is preferably attached in-line to the extracorporeal tubing on the arterial side of the dialyzer. The blood chamber provides a viewing point for optical sensors during the hemodialysis procedure. Multiple wavelengths of visible and infrared light are directed through the blood chamber and the patient's blood flowing therethrough, and a photodetector detects the resulting intensity of each wavelength. The preferred wavelengths are about 810 nm (e.g. 829 nm), which is substantially isobestic for red blood cells, and about 1300 nm, which is substantially isobestic for water. A ratiometric technique implemented in the corresponding controller, substantially as disclosed in U.S. Pat. No. 5,372,136 entitled “System and Method for Non-Invasive Hematocrit Monitoring”, which issued on Dec. 13, 1999 and is assigned to the assignee of the present application, uses this information to calculate the patient's hematocrit value in real-time. The hematocrit value, as is widely used in the art, is the percentage determined by dividing the volume of the red blood cells in a given whole blood sample by the overall volume of the blood sample. The system can also measure, optically, the oxygen saturation level in the blood flowing into the dialyzer. The preferred wavelength for measuring oxygen saturation levels are about 660 nm and about 810 nm.
In a clinical setting, the actual percentage change in blood volume occurring during hemodialysis can be determined, in real-time, from the change in the measured hematocrit. Thus, an optical blood monitor, such as the CRIT-LINE® monitor, is able to non-invasively monitor not only the patient's hematocrit level but also the change in the patient's blood volume in real-time during a hemodialysis treatment session. The ability to monitor real-time change in blood volume facilitates safe, effective hemodialysis and patient fluid management.
The blood chamber used in the current system comprises a molded body made of clear medical grade polycarbonate. The chamber body along with the tube set are replaced for each patient at each treatment. As mentioned, the blood chamber is normally attached in line to the extracorporeal tubing on the arterial side of the dialyzer. The most common area to experience leaks is where the blood chamber seats onto the dialyzer.
The blood chamber provides a flat and generally circular, internal blood flow cavity, as well as two circular viewing lenses: one being integrally molded with the body of the polycarbonate blood chamber and the other being welded into place into the body. The distance between the blood chamber lenses must be constant and maintained within the tight tolerances in manufacturing for calibration to be accurate and repeatable. An inlet port and channel communicate through a first opening into the flat and generally circular internal blood flow cavity, and the outlet port and channel communicate through a second opening. The first port and channel and second port and channel are in axial alignment through the diameter of the internal blood flow cavity. The inlet port is can be bonded to a tube set or terminate in a luer lock fitting, whereas the outlet port includes a fitting such as a luer lock type fitting intended for connection to a dialyzer blood filter. The attendant must be careful to properly seat the luer lock fitting on the port for the arterial side of the dialyzer in order to avoid leaking. The photoemitters and photodetectors for the optical blood monitor are clipped into place on the blood chamber over the lenses. The blood chamber is molded with a moat around the flat viewing region in the blood flow cavity between the viewing lenses. The moat holds a relatively thick layer of blood, and helps to attenuate ambient light and light piping inaccuracies.
The state of the flow of blood through the viewing area is quite important in order to obtain accurate, robust measurements. Laminar flow is not typically desirable. For this purpose, present day blood chambers include posts upstream of the viewing area to create eddy currents and mix the blood. This is more important at low velocities than at high velocities. Even though it is important to mix the blood and maintain homogeneity as it flows through the blood chamber, it is also important that the flow through the blood chamber not create hemolysis (i.e., rupture blood cells).